1. Field of the Invention
The present invention relates generally to improvements in optical communication systems. More particularly, the present invention relates to optical communication using FM to AM conversion induced by fiber chromatic dispersion.
2. Description of the Prior Art
Optical communication typically involves transmitting high bit rate digital data over silica glass fiber by modulating a laser or other optical source. Glass fibers have a very broad bandwidth, on the order of 40,000 THz, and can therefore in theory support total data rates on the order of 20,000 Tbits/sec. However, the practical fiber transmission capability is limited by system constraints, among the most important of which are the chromatic dispersion and nonlinearities of the optical fiber itself. Although optical fiber also attenuates the transmitted signal, at a rate of about 0.2 dB per km, the development of erbium-doped fiber amplifiers (EDFAs) has essentially eliminated fiber attenuation as an obstacle to achieving longer transmission distances.
Chromatic dispersion, often simply called dispersion, refers to a phenomenon in which the speed of an optical signal through the fiber varies as a function of the optical signal frequency or wavelength in standard single-mode fibers. For wavelengths below about 1.3 .mu.m, longer wavelengths travel faster than shorter ones, and the resulting dispersion is commonly referred to as normal dispersion. Above 1.3 .mu.m, shorter wavelengths travel faster than longer ones, and the dispersion is referred to as anomalous dispersion. Dispersion is typically expressed in units of picoseconds per kilometer-nanometer (ps/km-nm), indicating the number of picoseconds a pulse with a bandwidth of 1 nanometer will spread in time by propagating over 1 kilometer of fiber.
One important fiber nonlinearity is the Kerr effect, in which the index of refraction increases with the intensity of the applied optical signal. Changes in the fiber index of refraction modulate the phase of a signal passing through the fiber and thereby impose a frequency chirp which redistributes the signal frequency spectrum. This phenomenon is known as self-phase modulation in single channel systems in which the optical signal modulates itself. In multi-channel systems, in which one signal causes modulation of other signals, the phenomenon is referred to as either cross-phase modulation or four-photon mixing. Lower frequencies are placed at the leading edge of an optical signal pulse and higher frequencies are placed toward the trailing edge. Changes in frequency distribution are translated to phase modulation by the fiber dispersion. Chromatic dispersion and the Kerr effect therefore both lead to increasing optical signal distortion as a function of transmission distance. For long distance communication over optical fiber, therefore, dispersion and nonlinearities must be controlled, compensated or suppressed.
A dispersion and nonlinearity control technique, currently used in terrestrial and transoceanic optical fiber transmission, is electronic regeneration. Repeaters are spaced at appropriate locations along the transmission path to electronically detect, regenerate and retransmit the optical signal before the signal distortion becomes excessive. Electronic regeneration, however, limits the maximum achievable data rate to that of the electronic hardware, rather than that of the wider bandwidth optical fiber. In addition, repeaters are expensive to build and maintain, do not permit flexible system upgradability, and must be spaced at relatively short intervals along the fiber to effectively control optical signal distortion.
A number of repeaterless compensation techniques have also been developed. One such technique involves solitons, which are optical signal pulses having a well-defined amplitude, pulse width and peak power for a given anomalous dispersion value, such that self-phase modulation due to the Kerr nonlinearity and anomalous chromatic dispersion interact to stabilize the pulse shape. A soliton maintains its shape due to this interplay between dispersion and nonlinearity, and can therefore travel greater distances without regeneration. However, soliton systems also suffer from a number of significant drawbacks, including timing jitter and the need for sliding frequency filters to extend the bit rate-distance product beyond approximately 100 Tbits/s-km.
Another demonstrated compensation technique makes use of midsystem optical phase conjugation to compensate for first order dispersion. Because the phase conjugate of an optical pulse is a time reversal of the pulse, midspan optical phase conjugation allows the first order chromatic distortion of the first half of a fiber span to be eliminated by the identical distortion produced as the conjugated signal propagates along the second half. See A. Yariv, D. Fekete and D. Pepper, "Compensation for channel dispersion by nonlinear optical phase conjugation", Optics Letters, vol. 4, pp. 52-54, 1979, K. Kikuchi and C. Lozattanasane, "Compensation for Pulse Waveform Distortion in Ultra-Long Distance Optical Communication Systems by Using Nonlinear Optical Phase Conjugation," 1993 Technical Digest Series Volume 14, Conference Jul. 4-6, 1993, Yokohama, Japan. Midsystem optical phase conjugation has extended the bit rate distance product achievable in the anomalous dispersion region at 1.5 .mu.m wavelength of the conventional single mode fiber which makes up much of the world's existing fiber communication channels See A. Gnauck, R. Jopson and R. Derosier, "10 Gb/s 360 km Transmission over Dispersive Fiber Using Midsystem Spectral Inversion", IEEE Photonics Technology Letters, vol. 5, no. 6, June 1993.
Each of the above discussed techniques involve increasing fiber optic transmission distance by controlling or compensating dispersion and/or nonlinearities. However, it has also been recognized that chromatic dispersion produces an FM to AM conversion effect which can facilitate bit detection and thereby extend transmission distance without controlling or compensating dispersion. The dispersion causes shifting of adjacent signal components of different wavelengths, resulting in either energy voids or energy overlaps at the bit transitions. Constructive interference in an overlap causes a positive peak in the optical signal, while a void produces a negative peak. These positive and negative peaks represent an AM signal which may be detected to reproduce the original bit stream. The peaks are readily detected even when corresponding FM or AM data would have been excessively distorted by fiber dispersion effects. See E. Bochove, E. de Carvallo and J. Filks, "FM-AM conversion by material dispersion in an optical fiber," Optics Letters, Vol 6, No. 6, pp. 58-60, February 1981. It is therefore possible to go beyond the linear dispersion limit for either FM or AM modulated systems alone.
Demonstrated optical communication techniques using FM to AM conversion have an upper limit, for 10 Gbit/sec transmission over fiber with a dispersion of 17 ps/km-nm at 1.5 .mu.m wavelengths, of about 151 km without in-line amplification, and 204 km with one in-line optical amplifier. See B. Wedding and B. Franz, "Unregenerated Optical Transmission at 10 Gbit/sec . . . " Electronics Letters, Vol. 29, No. 4, Feb. 18, 1993. Thus, a bit rate distance product of about 2 Tbit/s-km is possible using available FM to AM conversion techniques. The present upper limit is primarily due to the effect of dispersion on the energy voids and overlaps. In addition, the effects of nonlinearities in the fiber have not been taken into account in designing existing FM to AM conversion systems. Since the FM to AM conversion process itself is entirely linear, nonlinearities such as self-phase modulation due to the Kerr effect have not been thought to play a role.
As an alternative to fiber optic systems using regenerative repeaters, solitons or optical phase conjugation, FM to AM conversion is an important technique. It is particularly useful for existing terrestrial intercity fiber optic links, which presently are generally made up of fiber having a chromatic dispersion of about 17 ps/nm-km at 1.55 .mu.m. Important advantages of FM to AM conversion over other available techniques for increasing dispersion-limited transmission distance include reduced hardware complexity and system cost, as well as ease of implementation and maintenance. Furthermore, FM to AM conversion is better suited to most existing terrestrial links than either soliton transmission or optical phase conjugation. Soliton transmission over terrestrial fiber would require prohibitively high optical signal amplitudes to generate sufficient Kerr nonlinearity to offset 17 ps/nm-km of dispersion. Optical phase conjugation performs best over fiber which has normal, or negative, dispersion values at the optical signal wavelengths. In addition, the spacing of fiber amplifiers in a terrestrial link is often dictated by terrain, and may thus fail to satisfy the lossless line approximation required for effective dispersion compensation using phase conjugation. Since soliton transmission or phase conjugation compensation may not be suitable, and available FM to AM techniques are limited to a transmission distance of about 200 km at 10 Gbits/sec, regenerative repeaters typically must be used in intercity fiber links covering distances greater than 200 km.
As is apparent from the above, a need exists for an improved optical communication systems based on FM to AM conversion. The improved system should take advantage of fiber nonlinearities in order to stabilize the energy voids and overlaps resulting from FM to AM conversion. Furthermore, the improved system should provide a substantial increase in bit rate distance product over presently available systems, without significant additional design, hardware or maintenance costs.